115 research outputs found
Optical Pulse Dynamics in Active Metamaterials with Positive and Negative Refractive Index
We study numerically the propagation of two-color light pulses through a
metamaterial doped with active atoms such that the carrier frequencies of the
pulses are in resonance with two atomic transitions in the
configuration and that one color propagates in the regime of positive
refraction and the other in the regime of negative refraction. In such a
metamaterial, one resonant color of light propagates with positive and the
other with negative group velocity. We investigate nonlinear interaction of
these forward- and backward-propagating waves, and find self-trapped waves,
counter-propagating radiation waves, and hot spots of medium excitation.Comment: 9 pages, 6 figure
Controlling pulse propagation in optical fibers through nonlinearity and dispersion management
In case of the nonlinear Schr\"odinger equation with designed group velocity
dispersion, variable nonlinearity and gain/loss; we analytically demonstrate
the phenomenon of chirp reversal crucial for pulse reproduction. Two different
scenarios are exhibited, where the pulses experience identical dispersion
profiles, but show entirely different propagation behavior. Exact expressions
for dynamical quasi-solitons and soliton bound-states relevant for fiber
communication are also exhibited.Comment: 4 pages, 5 eps figure
Averaging For Solitons With Nonlinearity Management
We develop an averaging method for solitons of the nonlinear Schr{\"o}dinger
equation with periodically varying nonlinearity coefficient. This method is
used to effectively describe solitons in Bose-Einstein condensates, in the
context of the recently proposed and experimentally realizable technique of
Feshbach resonance management. Using the derived local averaged equation, we
study matter-wave bright and dark solitons and demonstrate a very good
agreement between solutions of the averaged and full equations.Comment: 6 pages, 5 figures, in pres
Nonlinearity Management in Higher Dimensions
In the present short communication, we revisit nonlinearity management of the
time-periodic nonlinear Schrodinger equation and the related averaging
procedure. We prove that the averaged nonlinear Schrodinger equation does not
support the blow-up of solutions in higher dimensions, independently of the
strength in the nonlinearity coefficient variance. This conclusion agrees with
earlier works in the case of strong nonlinearity management but contradicts
those in the case of weak nonlinearity management. The apparent discrepancy is
explained by the divergence of the averaging procedure in the limit of weak
nonlinearity management.Comment: 9 pages, 1 figure
Stochastic Pulse Switching in a Degenerate Resonant Optical Medium
Using the idealized integrable Maxwell-Bloch model, we describe random
optical-pulse polarization switching along an active optical medium in the
Lambda-configuration with disordered occupation numbers of its lower energy
sub-level pair. The description combines complete integrability and stochastic
dynamics. For the single-soliton pulse, we derive the statistics of the
electric-field polarization ellipse at a given point along the medium in closed
form. If the average initial population difference of the two lower sub-levels
vanishes, we show that the pulse polarization will switch intermittently
between the two circular polarizations as it travels along the medium. If this
difference does not vanish, the pulse will eventually forever remain in the
circular polarization determined by which sub-level is more occupied on
average. We also derive the exact expressions for the statistics of the
polarization-switching dynamics, such as the probability distribution of the
distance between two consecutive switches and the percentage of the distance
along the medium the pulse spends in the elliptical polarization of a given
orientation in the case of vanishing average initial population difference. We
find that the latter distribution is given in terms of the well-known arcsine
law
Instabilities of dispersion-managed solitons in the normal dispersion regime
Dispersion-managed solitons are reviewed within a Gaussian variational
approximation and an integral evolution model. In the normal regime of the
dispersion map (when the averaged path dispersion is negative), there are two
solitons of different pulse duration and energy at a fixed propagation
constant. We show that the short soliton with a larger energy is linearly
(exponentially) unstable. The other (long) soliton with a smaller energy is
linearly stable but hits a resonance with excitations of the dispersion map.
The results are compared with the results from the recent publicationsComment: 20 figures, 20 pages. submitted to Phys. Rev.
English textbooks for Russian students: Problems and specific features
© 2017, Association for Social Studies Educa. All rights reserved. The research identifies the complexity level of eight texts from Spotlight 11 used in Russian TEFL to prepare students for National Unified Exam in English and assess their reading skills. The results of the analyses conducted with the help of T.E.R.A., an automated text processor, prove that all texts fell within the range of 6 – 9 Flesch-Kincaid grade levels which correspond to the English language proficiency of the target audience. We also revealed the absence of a clear progression in difficulty across the eight texts in the continuum which may cause unpredictable test results and contribute to demotivation of students. The results also show that the indices of narrativity, syntactic simplicity, word concreteness, referential cohesion and deep cohesion measured with T.E.R.A. do not grow but fluctuate across the continuum of the texts either. Aiming at selecting authentic texts with steadily growing complexity of each of the above mentioned parameters, we recommend to incorporate the suggested algorithm of text analysis into TEFL practice in Russia. T.E.R.A. is viewed by the authors as a tool able to provide educators with a solid foundation to select texts, develop curriculum, design assessment tasks and otherwise address academic needs of a target audience
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